我在/var/log/auth.log（Ubuntu 15.04）中找到了这一行
Aug 6 20:46:52 <> systemd-logind[641]: Power key pressed.
Aug 6 20:46:52 <> systemd-logind[641]: Powering Off...
Aug 6 20:46:52 <> systemd-logind[641]: System is powering down.
Aug 6 20:46:53 <> sshguard[1440]: Got exit signal, flushing blocked addresses and exiting...
Aug 6 20:46:53 <> sshd[1419]: Received signal 15; terminating.
这是什么意思？谁或什么关闭我的实例？